This invention relates to a process and apparatus for controlling an infinitely variable transmission in a manner that simulates a stepped transmission.
In known stepped transmissions, the transmission ratio jumps must be determined within the framework of the transmission layout, and can be optimal only for one driving situation. The transmission layout is therefore based on an average driving situation. A transmission layout obtained in this manner cannot be optimal in deviating driving situations, for example, during uphill or downhill driving, driving with a load or driving in traffic jams. Thus, the layout of the first gear is usually selected such that a loaded vehicle can also still start on steep slopes. However, in traffic jams or during stop-and-go traffic, this leads to frequent shifting operations because the layout of the first gear is too short for this situation and the driving engine rapidly reaches its rotational speed limit.
German Patent Document DE-A1 41 20 540 discloses an infinitely variable transmissions which simulates a stepped transmission in that transmission ratios are fixedly defined. Furthermore, German Patent Document DE-A1 42 39 133 discloses an arrangement which simulates an automatic multi-step transmission by means of an infinitely variable transmission while also defining fixed transmission ratios. The above-mentioned problems also occur here.
It is an object of the present invention to provide an improved transmission control of the generic type described above, which adapts the starting gear or the starting transmission ratio to a "low speed" driving situation.
This object is achieved by the control method and apparatus according to the invention, which includes a device for recognizing driving situations in which the transmission ratio jump to the next selected gear will be too large, as well as a device for adapting the presently selected preset transmission ratio in a manner which reduces such jump. That is, the recognition function activates the adapting function as soon as conditions exist which indicate a state of "low speed driving". The adapting device then reduces the transmission ratio from the current (highest) preset transmission ratio (that is, the "lowest gear") , starting at a predetermined engine speed at which the driving engine has built up a sufficient drive torque. (As noted hereinafter, the "transmission ratio" connotes a ratio of engine speed or transmission input speed divided by transmission output speed. Accordingly, a reduction of the transmission ratio changes the transmission ratio in the direction of what is conventionally referred to as the next higher "gear".)
This arrangement substantially improves the drivability of the highest preset transmission ratio (that is, the transmission ratio that corresponds to a "first gear") and, at the same time, reduces the shifting frequency, because the highest preset transmission ratio can be utilized over a larger speed range. Since the invention can be used in manually as well as automatically shifted transmissions, in the recognized driving situations the transmission ratio is adapted to the existing operating conditions of the vehicle without any loss of the typical characteristics of a stepped transmission.
In one embodiment of the invention, low speed driving is recognized when both the position and the adjusting speed of a power control element (for example, of an accelerator pedal or of a throttle valve) remain below preset limits. Alternatively, it is determined for this purpose whether the driving speed as well as the adjusting speed of the power control element remain below preset limit values.
In a further embodiment of the invention, the adapting function is triggered in response to the detection of a "starting operation", that is, for example, when the conditions indicated above are met immediately after the recognition of a vehicle stop (driving speed=0). By this type of recognition of low speed driving, a multitude of driving situations are recognized which have in common that driving occurs in a lower speed range for an extended period of time. This applies, for example, to both stop-and-go traffic and to door-to-door delivery traffic.
For adaptation of the highest preset transmission ratio (the lowest "gear"), in another embodiment of the invention, the transmission ratio is selected so that the rotational input speed of the transmission (or the rotational speed of the driving engine) remains constant. This adjustment of the transmission ratio is implemented as soon as the rotational input speed of the transmission reaches a minimum value, which is selected as a function of the throttle valve position, in order to thus adjust a rotational input speed level which corresponds to the throttle valve position, and therefore also an engine power level. Adjustment of the transmission ratio will end when the transmission ratio has reached a preset lower limit value, as described hereinafter.
In an alternative embodiment, the transmission ratio is adapted according to a preset function, depending on the rotational input speed of the transmission or on the driving speed of the vehicle. In this case also, the adjustment is started as soon as the rotational input speed reaches a preset value. This value of the rotational input speed need not be fixed, but can be adapted to the operating conditions. The adjustment is again terminated when a limit value for the transmission ratio is reached. Advantageously, the function for adjusting the transmission ratio is selected such that a limit value for the rotational input speed is reached at the same time when the limit value for the transmission ratio is reached. The limit value for the rotational input speed is advantageously selected slightly below the rotational input speed at which a transmission ratio change is automatically triggered, so that the end of the transmission ratio adjustment does not coincide with the change of the transmission ratio.
The lower limit value at which adjustment of the transmission ratio ends is advantageously selected so that a set proportion of the transmission ratio jump from the presently selected preset transmission ratio to the next lower preset transmission ratio is maintained between the adjusted transmission ratio to the next lower preset transmission ratio. In this manner, it is ensured that, upon change to the next lower preset transmission ratio, another transmission ratio jump takes place and thus the character of a stepped transmission is maintained.
The invention may be implemented in the form of a program, using a microprocessor-operated programmable control device. Of course, it is also possible to implement the invention by means of known discrete circuits elements, such as differential amplifiers. Thus, the invention relates to an operating range which can be implemented in the form of a control device-as well as of a control process, and the illustrated effects and advantages apply equally to the construction of the control device as well as the implementation of the control process.
The above-described effects and advantages relate to a manually shifted as well as to an automatically shifted simulation of a stepped transmission by means of an infinitely variable transmission.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.